The present invention relates to a bulky sheet material comprising nonwoven fabric having a large number of protrusions.
Japanese Patent 3,131,557 discloses wrinkled nonwoven fabric composed of a first fiber layer comprising heat shrinkable fiber and heat bondable fiber whose melting point is lower than the shrinkage starting temperature of the heat shrinkable fiber and a second fiber layer comprising heat non-shrinkable fiber superposed on one side of the first fiber layer, the first and second fiber layers being joined by thermal fusion bonding in a stripe pattern. The fusion bonded joints are depressions, and the second fiber layer forms a great number of streaky wrinkles (ridges) between the fusion bonded joints. The wrinkled nonwoven fabric is produced by superposing the first fiber layer and the second fiber layer, joining the two layers by fusion bonding at a temperature lower than the shrinkage starting temperature of the heat shrinkable fiber, and blowing hot air at or above the shrinkage starting temperature to shrink the shrinkable fiber. Because shrinking the heat shrinkable fiber is effected at temperatures higher than the melting point of the resin making the heat bondable fiber, it is accompanied by fusion of the heat bondable fiber, which results in stiffness of the resulting nonwoven fabric. Further, the bonding strength between the two layers is limited because the thermal fusion bonding of the first and second fiber layers relies on the heat bondable fiber which is contained in the first fiber layer in a proportion of 30 to 50% by weight. With the limited bonding strength, the fusion bonded joints are apt to loosen when the first fiber layer shrinks or while the resulting nonwoven fabric is further processed or in use, which results in an obscure pattern of ridges or a failure to form a desired pattern of ridges.
JP-A-9-3755 discloses nonwoven fabric with a textured surface which is composed of a first fiber layer containing thermally shrunken fiber and a second fiber layer containing non-shrinkable short fiber superposed on one side of the first fiber layer, the first and second fiber layers being joined by partial heat fusion bonding. The second fiber layer has raised portions making regular protrusions between the fusion bonded joints by heat shrinkage of the first fiber layer. The nonwoven fabric is produced by superposing the first and second fiber layers and passing the two layers under an embossing roll thereby joining them together in parts and simultaneously causing the first fiber layer to shrink. Because heat is hardly conducted throughout the first fiber layer from the embossed parts, it is difficult to shrink the heat shrinkable fiber of the first fiber layer to a high shrinkage percentage for sufficiently making the second fiber layer protrude. Where the fibers constituting the second fiber layer are in an unbonded state, the network of the fusion bonded fibers is insufficient for forming protrusions having high shape retention. As a result, the protrusions are collapsed easily and fuzz up easily.